1. Field of the Invention
The present invention relates to fuel cell electric power plants having black start capability, and black start systems and methods for fuel cell power plants.
2. Description of the Related Art
Fuel cells are known in the art. Fuel cells electrochemically react a fuel stream comprising hydrogen and an oxidant stream comprising oxygen to generate an electric current. Fuel cell electric power plants have been employed in transportation, portable and stationary applications.
Stationary and portable applications include distributed power generation, back-up power, peak power, and uninterruptible power supply (UPS) systems. Distributed power generation relates to providing electrical power to residential, commercial and/or industrial customers instead of, or as a supplement to, the utility power grid. Power plants in such applications typically operate continuously. They are particularly suited to situations where the power grid is not available or sufficiently reliable. Peak power systems are intended to supplement the power grid, providing electrical power intermittently during periods of peak use when sufficient grid power may not be available or when the rate charged by the utility increases. Back-up power and UPS systems provide electrical power during periods when the power grid, or other primary power source, is unavailable. Fuel cell power plants may also be employed in transportation applications, including cars, trucks, buses, trains, ships and aircraft.
In addition to one or more fuel cell stacks, fuel cell electric power plants also comprise associated operating, monitoring and control systems, generally referred to as the balance of plant. The balance of plant typically includes reactant supply and power conditioning systems, and may also include various other systems for reactant humidification, temperature regulation, and operational monitoring, for example. The balance of plant usually also includes a control system for controlling the operation of the power plant.
These systems include various electrical devices that require an electrical power source for operation, including blowers, compressors, regulators, sensors, powered valves and electronics, for example. Collectively, these devices represent an operational load that must be supplied with power before the fuel cell electric power plant can begin normal operation. Of course, once the fuel cell stack is producing a nominal output it can begin to power the operational load.
Known fuel cell electric power plants may employ an electrical storage device, such as secondary batteries or super capacitors, to power the operational load until the fuel cell stack is operating and can provide sufficient power to do so. The electrical storage devices may provide power to external loads as well. For example, U.S. Pat. No. 6,266,576 B1 (“Okada et al.”) discloses a fuel cell unit comprising a fuel cell, a fuel cell controller and a double-layer capacitor (or other electric energy storage means). To start the fuel cell unit the fuel cell controller receives electric energy from the double-layer capacitor or an external power source. Where the double-layer capacitor is the energy source, it is charged by the fuel cell prior to shutdown or refueling to ensure the double-layer capacitor has sufficient energy to power the controller on restart. Alternatively, the external power source may be used to charge the double-layer capacitor or supply the needed power to the controller directly. While this solution is suitable for certain applications, it does have some disadvantages.
For example, in some applications the power plant may be not be used for extended periods. Over time, such electrical storage devices lose electrical energy due to self-discharge, with the result that insufficient power is available to supply the operational load of the fuel cell stack, and the power plant will not start. Damaged or defective electrical storage devices may also fail to produce sufficient power for the operational load. Thus, without an external power supply the power plant will not be able to start. As another example, situations may arise in which the power plant is required to start but the electrical storage device, though functioning properly, has been discharged. This can occur when the reactant supply of the power plant is interrupted during operation, e.g., the power plant “runs out of gas”. In this situation the electrical storage device may be discharged supplying power to the external load, or may not have been adequately recharged before the fuel cell stack stopped producing power. Unfortunately, once reactant supply is re-established, the power plant will not be able to resume operation without an external power source to power the operational load. For back-up, portable and vehicular applications, in particular, where another external power source may not be available, this can be a significant problem.
It is desirable to have a fuel cell electric power plant that has black start capability, i.e., is able to start operation without requiring an external power source. The present invention addresses the disadvantages of conventional fuel cell power plants and provides further related advantages.